1. Field of the Invention
In general terms, the invention relates to seismic data processing and, more particularly, the present invention allows accessing information referred to as specular information after transformation of temporal seismic records in the depth domain. This depth migration is carried out by means of a prestack depth migration algorithm of Kirchhoff type for imaging an underground zone. These quantities can be used directly or as intermediaries for algorithms intended to update the velocity model of an underground zone, such as tomography algorithms.
2. Description of the Prior Art
Reflection shooting is widely used in oil exploration. This technique supplies temporal information relative to the subsoil from the information contained in the seismic waves propagated and reflected on the geological discontinuities of the medium. By approximation, the propagation and the reflection of the seismic waves are approximated by rays that are propagated in a complex velocity domain and are reflected on a reflector (geologic interfaces or various heterogeneities of part of the subsurface) according to Snell's law.
Seismic imaging methods use the kinematic information associated with seismic reflections, that is the arrival times of the waves, to determine a velocity representation of the subsoil.
From this information on the subsoil velocity, it is possible to change a temporal image of the subsoil into a depth image, by means of an algorithm referred to as depth migration algorithm. This technique first defines the geometric parameters of the desired depth image of the subsoil. This image is a set of points referred to as image points. Then, the associated temporal seismic information (amplitudes) is associated with each image point.
The following documents mentioned in the course of the description below illustrate the state of the art:    Stork, C., 1992, Reflection Tomography in the Postmigrated Domain: Geophysics, 57, 680-692,    Bishop, T. N., Bube, K. P., 1985, Tomographic Determination of Velocity and Depth in Laterally Varying Media: Geophysics, 50, No. 6, 903-923,    Ehinger, A., and Lailly, P., 1995, Velocity Model Determination by the SMART method, Part 1: Theory: 65th Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, pp. 739-742,    Bleistein, N., 1987, On the Imaging of the Reflectors in the Earth: Geophysics, 52 (7), 931-942,    Schneider, W. A., 1978, Integral Formulation for Migration in Two and Three Dimensions: Geophysics, 43, 49-76.
Prestack migration is a conventional method of processing seismic data. In general terms, the technique consists, in knowing the value of a wavefield at a known depth, at the surface for example, and a model of the wave propagation velocity distribution in the underground zone, in modelling the propagation of the source field and the back propagation of the recorded reflection data, and in seeking phase coherences between these two modelled fields.
If a correct subsoil velocity model is available, the depth migration constructs a depth image on which a structural interpretation of the subsurface can be achieved, by a geologist for example. This depth image of the subsoil is generally referred to as depth migrated image. In three dimensions, it is referred to as depth migrated cube.
In the opposite case, that is if no correct subsoil velocity model is available, depth migration allows, on the one hand, evaluation of the quality of a velocity model and, on the other hand, to produce information allowing this velocity model to be improved. The tomographic methods appear among the methods which allow use of this information resulting from depth migration to update the velocity model.
In the domain referred to as prestack domain, that is prior to summation of the temporal records from various offsets, there are two main tomography types: tomographies referred to as prestack depth tomographies and tomographies referred to as prestack time tomographies.
Thus, prestack depth tomographies (Stork, 1992) are based on the fact that, if the velocity model used for a prestack depth migration is correct, then the seismic events appear flat in the iso-x collections (sections at a given lateral position in the depth migrated cube). If this is not the case, the information contained in the iso-x collections, that is the variability as a function of the reflector image offset, is used in order to update the subsoil velocity model. However, to correctly use this information, it is necessary to know the coordinates of the sources and receivers that have constructively contributed to the image of the reflector at each point of the iso-x collection considered. Now, after depth migration, this information, referred to as specular, is unknown.
Prestack time tomographies (Bishop et al., 1985) come up against the same problem. In fact, in the case of complex structures, access to the input data of these methods, that is prestack traveltimes, can be difficult or even impossible directly in the time domain. In such cases, a departure will be made in the depth migrated domain, followed by a stage referred to as kinematic demigration in order to convert this information collected in the depth domain to temporal information (Ehinger and Lailly, 1995). Now, the demigration stage uses ray tracing from an image point considered in the prestack depth migrated domain to the specular sources and receivers which are here, again, postmigrated unknown quantities.
Thus, whether in the depth domain or in the time domain, access to the space coordinates of the specular sources and receivers of a seismic image is therefore necessary for the velocity model updating methods which use the information collected in the depth migrated domain, such as tomographic methods. Now, depth migration algorithms lose this information during migration.
Solutions are currently under development in the industry. These solutions, derived from the stationary phase theory, carry out different migrations using the same data and the same velocity model, but for different migration operators (Bleistein, 1987). More precisely, in addition to the conventionally calculated depth migrated cube, other cubes are calculated by weighting the migration operator itself by the desired quantities (for example the position of the sources and the position of the receivers). Then, by calculating the ratio of the conventional cube to the weighted cubes, an estimation of the desired specular quantities is obtained. This approach has two major drawbacks. First of all, this technique may be costly in calculation time costly insofar as several migrations (as many migrations as desired specular quantities) have to be performed. Besides, this approach can lead to erroneous specular quantity values when the signal-to-noise ratio is low, and also in zones where the amplitudes are low, which may occur in the case of complex structures.
the invention allows access to specular quantities after prestack depth migration of the temporal seismic records, quantities that are necessary notably for updating the velocity model of seismic waves.